EP2721388B1 - Method of determining the optimal wavelength for inspecting ophthalmic lenses - Google Patents

Description

Field of the Invention
• This invention relates to the inspection of ophthalmic lenses, particularly hydrogel contact lenses for missing material defects.
Background of the Invention
• Ophthalmic lenses, such as hydrogel contact lenses are formed, inspected, and packaged on manufacturing lines with minimal human intervention. During these processes, certain defects to those lenses occur and one common defect is missing material in a formed lens. Whether such missing material is the entire thickness of a contact lens, or just a portion of its thickness, lenses with such defects should be removed from the product that ultimately reaches end users.
• There are inspection methods, which find holes in ophthalmic lenses. However, given the variety of different types of contact lens materials that are on the market, lens makers often must inspect such lenses using radiation of different wavelengths. This is particularly true if one is looking for a missing material defect which is not a complete hole, such as a depression in the lens material. Typically the process of finding radiation of an optimal wavelength is a trial and error process. This trial and error method wastes a great deal of time and materials, and does not guarantee an optimal wavelength selection.
• The document US2002/0122172 A discloses such an inspection method
• Therefore, it is desirable to determine the optimal wavelength of radiation for the inspection of such missing material defects without a trial and error process. This desire is met by the foregoing invention.
Description of the Figures
• Fig. 1 Plot of % transmission over several thickness at several wavelengths.
• Fig. 2 Plot of theoretical Beer's Law calculation over the experimental data of Fig 1.
• Fig. 3 Plot of Contrast over several wavelengths.
Detailed Description of the Invention
• This invention provides a method of determining the wavelength of radiation which may be used to automatically inspect an ophthalmic lens of a maximum thickness for missing material of an amount between about zero and about a certain thickness, comprising steps as defined in Claim 1.
• As used herein the term "ophthalmic lens" refers to soft contact lenses, such as hydrogels which are made from monomers, macromers or prepolymers. Examples of such ophthalmic lenses include but are not limited to lenses made from the following generic formulations acofilcon A, alofilcon A, alphafilcon A, amifilcon A, astifilcon A, atalafilcon A, balafilcon A, bisfilcon A, bufilcon A, comfilcon, crofilcon A, cyclofilcon A, darfilcon A, deltafilcon A, deltafilcon B, dimefilcon A, drooxifilcon A, epsifilcon A, esterifilcon A, etafilcon A, focofilcon A, genfilcon A, govafilcon A, hefilcon A, hefilcon B, hefilcon D, hilafilcon A, hilafilcon B, hioxifilcon B, hioxifilcon C, hixoifilcon A, hydrofilcon A, lenefilcon A, licryfilcon A, licryfilcon B, lidofilcon A, lidofilcon B, lotrafilcon A, lotrafilcon B, mafilcon A, mesifilcon A, methafilcon B, mipafilcon A, narafilcon, nelfilcon A, netrafilcon A, ocufilcon A, ocufilcon B, ocufilcon C, ocufilcon D, ocufilcon E, ofilcon A, omafilcon A, oxyfilcon A, pentafilcon A, perfilcon A, pevafilcon A, phemfilcon A, polymacon, silafilcon A, siloxyfilcon A, tefilcon A, tetrafilcon A, trifilcon A, and xylofilcon A. More particularly preferred ophthalmic lenses of the invention are genfilcon A, lenefilcon A, comfilcon, lotrafilcon A, lotraifilcon B, and balafilcon A. The most preferred lenses include etafilcon A, nelfilcon A, hilafilcon, polymacon, comfilcon, galyfilcon , senofilcon , and narafilcon.
• The term thickness refers to the measurement of the ophthalmic lens from its front surface to its opposing back surface. A typical hydrogel contact lens has a thickness of about 60 µm to about 600 µm. For purposes of this invention, the thickness of the finished product is the "maximum thickness". In the methods of the invention radiation is transmitted through hydrogel contact lenses having a thickness of from about 200 µm to about 600 µm, preferably about 85µm to about 209 µm.
• The term "certain thickness" refers to the depth of a missing material defect which does not go through the entire maximum thickness of the ophthalmic lens. For example for an ophthalmic lens having a maximum thickness of about 350 µm, a certain thickness is any number from about 300 µm to about 50 µm. Preferably certain thickness is selected from the group consisting of 30 µm, 40 µm, 50 µm, and 60 µm.
• As used herein, the term "% transmittance" means the amount of radiation which reaches a spectrometer after its transmission through either a cuvette, an ophthalmic lens and a solution, or a cuvette and solution. In either case the non-limiting examples of solutions are deionized water and saline solution, preferably saline solution.
• In the method, the transmitted radiation may have wavelengths in the visible, ultraviolet, or infrared radiation. Visible radiation has wavelengths from about 390nm to about 700 nm, ultraviolet radiation has wavelengths from about 10 nm to about 390 nm, and infrared radiation has wavelengths from about 700 nm to about 3000 nm. It is preferred that radiation in the range of about 340 nm to about 550 nm is transmitted through the ophthalmic lenses.
• As used herein the term "contrast value" means the difference in transmission between the two different thickness of lens material.
• As used herein the "k" value is the constant found in the theoretical relationship known as Beer's Law. Beer's Law relates the percent of transmission of radiation through a material ("%T") to the thickness of the material ("t") and a constant ("k") (%T= 10 ^(2-kt)). Each wavelength has a particular k which may be calculated by known methods, such as regression fitting.
• The wavelengths found by this method may be used in a number of inspection techniques. Non-limiting examples of such techniques are disclosed in the following patents US Patents 6,882,411 , 6,577,387 , 6,246,062 ; 6,154,274 ; 5,995,213 ; 5,943,436 ; 5,828,446 ; 5,812,254 ; 5,805,276 ; 5,748,300 ; 5,745,230 ; 5,687,541 ; 5,675,962 ; 5,649,410 ; 5,640,464 ; 5,578,331 ; 5,568,715 ; 5,443,152 ; 5,528,357 ; and 5,500,732 ; all of which are incorporated herein in their entireties by reference.
Example
• Ten etafilcon A hydrogel lenses having center thicknesses from 93 µm to 252 µm were prepared. Each sample was placed in a cuvette having internal dimensions of 18.5 mm wide x 5.1 mm wide x 21.2 high (without a cap) which holds approximately 1650 mL of liquid with a cap. Light of wavelengths from 340 nm to 420 nm was shown through the lens/cuvette/saline solution and the percent transmission was obtained using a Perkin Elmer UV/VIS Lambda 18 spectrometer. The percent transmission versus wavelength for every lens thickness is plotted in Fig. 1. Fig. 2 overlays the theoretical calculation over the experimental data of Fig. 1 and illustrates that this material behaves in accordance with Beer's Law.
• For each wavelength the % transmittance for a cuvette/saline solution/lens having a thickness of 50 µm was subtracted from % transmittance for a cuvette/saline solution to give a first contrast value. For each wavelength the % transmittance for a cuvette/saline solution/lens having a thickness of 350 µm was subtracted from % transmittance for a cuvette/saline solution//lens having a thickness of 300 µm to give a second contrast value. The first and the second contrast values were compared at each wavelength and the lower of the two values was plotted against wavelength in Fig. 3. This figure shows that the highest peak occurs at around 375 nm, therefore, the best wavelength to determine a missing material defect of about 50 µm is 375 nm.

Claims (8)

1. A method of determining the wavelength of radiation which may be used to automatically inspect an ophthalmic lens of a maximum thickness for missing material of an amount between about zero and about a certain thickness, comprising:

   (a) measuring a percent of transmittance of radiation of several different wavelengths through ophthalmic lenses of several different known thicknesses;

   (b) calculating a k value for each wavelength of the several different wavelengths by regression fitting and confirming that transmission of light through the ophthalmic lenses follows Beer's Law, wherein Beer's Law is represented by: $$\mathrm{percent\; of\; transmittance}={10}^{\left(1-\mathit{kt}\right)},$$

   

   
   where t is the thickness of the ophthalmic lens;

   (c) subtracting the percent of transmittance at said certain thickness from the percent of transmittance in the absence of an ophthalmic lens at said several different wavelengths to give first contrast values;

   (d) subtracting the percent of transmittance at said maximum thickness from the percent of transmittance at said certain lens thickness at said several different wavelengths to give second contrast values;

   (e) comparing the first contrast values with the second contrast values at each wavelength and selecting the lowest contrast values at each wavelength and then plotting such lowest contrast values against wavelength; and

   (f) selecting the wavelength from the plot of step (e) at the highest peak for inspection of missing material defects.
2. The method of claim 1 wherein the maximum thickness is about 60 µm to about 400 µm.
3. The method of claim 1 wherein the maximum thickness is about 85 µm to about 209 µm.
4. The method of claim 1 wherein said certain thickness is a number from about 20 µm to about 100 µm.
5. The method of claim 1 wherein said certain thickness is a number selected from the group consisting of 30 µm, 40 µm, 50 µm, and 60 µm.
6. The method of claim 1 wherein said radiation of several different wavelengths are from about 340 nm to about 430 nm.
7. The method of claim 1 wherein said radiation of several different wavelengths are from about 340 nm to about 550 nm.
8. The method of claim 1 wherein said ophthalmic lenses are selected from the group consisting of etafilcon A, nelfilcon A, hilafilcon, polymacon, comfilcon, galyfilcon , senofilcon , and narafilcon.

Images